In modern medicine, non-invasive or minimal invasive methods are widely used for organ imaging. The aim of these methods is essentially to obtain detailed knowledge about the respective organ and its state without opening the body. In the computed tomography system mentioned initially, three-dimensional images of the interior of an object that is being examined are produced using an X-ray method. For this purpose, an X-ray radiation source rotates, as described, very quickly along a circumferential ring (normally in a so-called gantry housing) around the body of the patient, and passes radiation through the body during the process. A detector is in each case located opposite the X-ray radiation source and detects the X-rays, which have been attenuated by the body, on a position-resolved basis. Two-dimensional X-ray slice images can then be reconstructed from the image data recorded by the detector device, from which, finally, slice images and a three-dimensional image can be reconstructed.
In computed tomography, a fundamental distinction is drawn between two recording methods. In one method, the gantry is moved continuously forwards around the body in the longitudinal direction of the body relative to the patient (referred to in the following text as the z-direction) while the X-ray radiation source is being rotated, so that the X-ray radiation source revolves around the body in the form of a helix throughout the entire examination. This method is normally referred to as spiral scanning. A further method is so-called “sequential scanning”, as has already been mentioned in the introduction. In this case, the circumferential ring is kept at a fixed position while recording a specific slice plane.
Once the slice image has been completed at this position, then the circumferential ring is subsequently moved to the new position relative to the body, and a new recording is thus produced in an adjacent slice plane. In this case, the expression a relative movement of the circumferential ring with respect to the body of the patient is intended in the following text to mean not only a movement of the circumferential ring with respect to a patient whose position is fixed in space on an examination table or the like but also—as in the case of most equipment—a movement of the patient, with an examination table which can be moved by the circumferential ring in the z-direction, with a gantry housing being fixed in space.
In many cases, the spiral scanning method is used nowadays, with the X-ray emitter generally emitting continuously during the forward movement. On the other hand, this method, which necessarily results in a large overlap of image data and in which in consequence the tissue must also be irradiated more than once, leads to larger doses being applied. On the other hand, the sequential method has the advantage that a dose is in principle applied only when a recording is also actually being made of one specific slice image plane. In this case, at least half a revolution plus the beam angle resulting from the beam geometry of the X-ray beam are required for reconstruction, with the beam angle generally being in the order of magnitude of 50°.
In practice, a transition angle of about 30° is normally additionally used in order to avoid artifacts at the interfaces of the projection angle interval. In order to improve the time resolution and thus to improve the image quality, it is also possible for a segment such as this to be composed of segment elements, for example of two segments each having only one quarter of the rotation (in each case plus the equipment-dependent beam angle), which are recorded at the same examination table position.
However, one problem with all of these methods is to record organs which move rapidly. Specifically, the only image data which can be used for a sensible image reconstruction is that which in each case shows the organ in the same state. In one of the most important applications, heart recording, the filling phase or diastole is in this case preferably chosen as a phase in which the heart is relatively at rest, for display purposes. With a living human, this rest phase lasts for less than 100 ms, even when relaxed in a resting position. In consequence, only image data within a limited measurement time interval, at a time within this rest phase, can be used for image reconstruction. However, in addition, there are also specific recordings in which the heart is in fact intended to be recorded in the systolic phase, that is to say in maximum contraction.
At the moment, most cardiac CT examinations (heart computed tomography examinations) are carried out using a spiral recording process, with the recorded image data being selected (gated) retrospectively. For this purpose, an EKG of the patient is recorded at the same time, in parallel with the computed tomography recording. The permissible measurement time intervals within the respectively desired heart phases are determined on the basis of this EKG signal. The only image data which is then used for reconstruction is that originating from these permissible measurement time intervals. All the other data is generally ignored for image reconstruction of the heart. Although these examination methods have the advantage of a relatively short examination time, their disadvantage is a high dosage application.
In order to reduce the dose at least somewhat, spiral scanning methods are already currently in use, in which the tube current for the X-ray radiation source is modulated correlated with an EKG signal, with the dosage thus being reduced to approximately 20%, for example, in specific phases in which the heart definitively cannot produce any recordings that could be assessed. This remaining dose is just sufficient in order to use the image data obtained in this way for reconstruction if necessary, provided that no suitable image data is available from a spatial direction that is required for complete reconstruction.
In contrast, with the broad multi-row detectors which have recently become available and have a plurality of detector rows in the z-direction, sequential scanning methods are also now of interest for cardio-CT examinations. In order to ensure for this purpose that the dose is as low as possible, it is highly worthwhile triggering the recording with the aid of a cycle signal which represents the movement cycle of the organ, for example with the aid of the EKG signal in the case of a heart recording. The EKG can then be used to define the next measurement time interval, so that radiation is applied only within this time interval. However, this method has the disadvantage that it works sufficiently well only if the EKG is highly uniform. In most cases, however, the heart rate varies during recording.
In addition, arrhythmic movements of the heart also frequently occur. This is because the patient is virtually unavoidably in a stress situation during the measurement, and in fact these are generally patients who are actually being investigated because their heart behavior is abnormal.
In none of these cases is it certain that the measurement time interval defined in advance on the basis of the heart rate will adequately match the phase of the heart to be recorded. This can be determined only while the recording is actually being made. This means that it is not possible to decide until after an X-ray dose has been applied whether this was justified at all and whether the image data measured immediately before this can be assessed.
In general, particularly in the case of patients with arrhythmic EKGs, much of the data is therefore rejected, so that the dose was admittedly applied, but was not used. In the end, this then leads on the one hand to the examination times being longer than in the case of traditional spiral scans and the radiation dose in fact coming close to that of spiral scans, because of the recordings which cannot be used. For this reason, dose-reducing sequential scanning is generally not used—except in the case of patients with a very rhythmic EKG, and spiral scanning is carried out instead of this.
In order to keep the unnecessarily applied dose as small as possible, US 2004/0077941 A1 proposes a method which, inter alia, can also be used for a helical scan, and in which an EKG of the patient is recorded in advance during a test phase. This EKG is analyzed in order to find a plurality of successive “normal” QRS complexes. A representative QRS complex, and therefore a representative RR interval for the patient, are then determined on the basis of these QRS complexes. The length of a measurement time interval which will normally fit well into a desired phase of the representative QRS complex is then defined with the aid of the representative RR interval.
During a subsequent computed tomography measurement, an EKG which is recorded at the same time is used to determine whether this has any discrepancies from the representative QRS complex, in order in this way to detect arrhythmia. If an arrhythmia is detected during a measurement time interval, then the X-ray tube is switched off prematurely, and the measurement data is rejected. The process then waits for the next suitable QRS complex into which the defined measurement time interval fits. However, if the EKG is highly arrhythmic, this unfortunately also leads to a very long measurement time and to a large number of irradiation time periods which cannot be used, although they are shorter.